Ultra high frequency antenna



Jan. 30, 1951 s, WEHNER 2,539,680

ULTRA HIGH FREQUENCY ANTENNA Filed Nov. 26, 1945 3 Sheets-Sheet 1 INVENTOR Mam ATTORNEY Jan. 30, 1951 R. s. WEHNER 2,539,680

ULTRA HIGH FREQUENCY ANTENNA Filed Nov. 26, 1945 3 Sheets-Sheet 2 I INVENTOR ATTORNEY R. s. WEHNER ULTRA HIGH FREQUENCY ANTENNA Jan. 30, 1951 3 Sheets-Sheet 5 Filed Nov. 26, 1945 INVENTOR RObGIZ e/Z Miner ATTORNEY .inthe wall of a building or tower.

Patented Jan.

ULTRA HIGH FREQUENCY ANTENNA Robert S. i Vehner, Port J efierson, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application November 26, 1945, Serial No. 630,925

2 Claims.

The present invention relates to ultra high frequency antennas and more particularly to such antennas particularly adapted for use on airplanes.

An object of the present invention is the provision of an ultra high frequency aircraft antenna having zero aerodynamic drag.

Another object of the present invention is the provision of an aircraft antenna capable of being entirely contained within the skin of the airplane.

A further object of the present invention is the provision of an antenna, as aforesaid, which has broad-band impedance characteristics.

Still a further object of the present invention is the provision of the antenna, as aforesaid, which is capable of more than 20% band width when operated in conjunction with lengths of commercially available coaxial transmission line.

Still a further object of the present invention is the provision of ultra high frequency aircraft antenna which has radiation characteristics making the antenna suitable for either vertically or horizontally polarized radiation.

Still another object of the present invention is the provision of an ultra high frequency aircraft antenna which has physical dimensions small enough to be practical at frequencies higher than 300 megacycles per second, even on the smallest aircraft.

Still a further object of the present invention is the provision of an ultra high frequency aircraft antenna of simple physical design of great inherent mechanical strength.

Still another object of the present invention is the provision of an ultra high frequency aircraft antenna which may be installed in aircraft without in any way weakening the structure or skin of the ship.

The foregoing objects and others which may appear from the following detailed description are attained by providing a cylindrical or semi- I cylindrical sleeve radiator mounted coaxially in a material set flush in the skin of the ship.

While intended primarily for use on aircraft, it will be evident from the following description that the use of this antenna is readily extended to other vehicles and to fixed installations such as type of installations, where the restrictions im- In the latter posed on physical dimensions are not as severe as on aircraft, use of the antenna can be extended to frequencies much lower than 300 megacycles per second.

The present invention will be more fully understood by reference to the following detailed de-- scription in which:

Figure 1 shows in perspective an embodiment of the present invention, while Figures 2, 3 and 4 are curves illustrating the impedance characteristics of an antenna simiiar to that of Figure l with different feeding means connected to the antenna.

Figure 5 is a schematic diagram illustrating the arrangement of an elemental transmission line matching section for the antenna of Figure 1, while Figure 6 is a plan view partly in section of the antenna of Figure 1, showing the mechanical details of the matching section in more detail, and

Figures 7 and 8 are field strength patterns of the antenna of Figure 1.

Referring now to Figure 1, reference numeral if! identifies the skin of a ship; that is, it isthe metallic outer casing of the fuselage of an airplane or it may be the upper or lower surface of one of the wings of the airplane. In the conductive sheet If; is cut a rectangular aperture i2. A semi-cylindrical cavity is formed back of aperture l2 by a curved conductive rear wall [4 and semi-circular end plates l6 and I8. Aperture I2 is preferably closed by a thin window I2 of polystyrene or other low loss dielectric constant ma terial so arranged flush with the outer surface of conductive sheet Ill. Thus, the entry of rain, snow or dust into the cavity is prevented and the electrical characteristics of the cavity maintained constant. One of the end walls, that is, end wall i8 has an aperture therein through which passes the outer sleeve 20 of a sleeve fed coaxial antenna. The upper half of the radiator is formed by rod radiator H which may be directly connected to the inner conductor 23 of coaxial transmission line TL. The outer shell 24 of transmission line TL is directly connected to sheet i8 and also to sleeve 20. Dimensions of the system which have been successful are indicated in the drawing and are tabulated as follows:

Cavity length, approximately wavelength. Cavity radius, 1% wavelength to wavelength,

depending upon whether a cylindrical or semicylindrical radiator is used. Antenna length, approximately wavelength. Sleeve length, approximately wavelength.

All of the wavelengths referred to being that of the lowest frequency of the desired pass band, the dimension applying to an air-filled cavity.

For an antenna designed to cover the range of from 750 to 1,000 megacycles per second, the cavity dimensions may be as follows:

Referring now to Figure 2 which is a curve showing the impedance characteristics of an antenna in a cylindrical cavity using a straight axial stub antenna, 7.6 centimeters long, and 0.9

centimeter in diameter, curve 30 illustrates the variation in input impedance as the input frequency is varied from 750 megacycles/sec. to- 1,000 megacycles/sec. The region of less than 2:1 standing wave ratio on a 50-ohm line is delineated by dotted circle 3|. It will be noted that the entire length of curve 30 is outside of the circle 3! showing that at no point is it possible to attain less than 2:1 standing wave ratio on a 50-ohm line with a simple stub antenna.

By adding a coaxial sleeve 3.8 centimeters long and 2.4 centimeters in diametera-round the lower end of the stub radiator the impedance characteristics shown in Fig-ure 3 are obtained. Curve 40, it will 'be noted, now passes through the circle 3| limiting the region of less than 2:1 standing waveratio in the region between 800 to 880 megacycles per second. Thus, there is a band width with less than 2:1 standing wave ratio, with the antenna fed directly from 50-ohm line, without recourse to matching circuits.

By further modifying the antenna to include a two element-matching section in the sleeve, the

characteristics in Figure '4 are obtained. Here curve 50 is within-the dot-ted circle 3| for nearly sleeve surrounding the lower-end of the radiator 2| results in the impedance level being raised, the resonant resistance now being '40 ohms as "compared to ohms for the stub antenna above. The rate of variation of reactance with frequency, as will be noted from the slope of curve '40, is decreased while'the-resonance is shifted upwardly some 40 megacycles toward higher frequencies.

The matching section which results in the improved characteristics shown inFigure 4 over those shown in Figure '3 is shown diagrammati- 'cally in Figure '5, while the structural details are shown in'more detail in Figure 6. 'In Figure-5 the main transmission line from the source (not shown) is identified by reference letters TL, the

said transmission line being connected to a series matching section 55 by means of a conventional type'of .line COIl'Il80tOI 15fi. -In the case :of, a-

dhmctransmissionilinerthe series .section may have an impedanceof 60ohms andsbe 81;;electrical degrees in lengthzat'760 megacycles'. The

.other end ofthe .series section 'is;connec ted .to

the antennasindicated diagrammatically hereby .1 d.resistor At the ,junction betweenthe' series section and load LR there is connected a short circuited shunt section 5! acting as an anti-resonant shunt. Since the antenna is series resonant, the shunt section must be anti-resonant so that the rate of antenna input reactance with change of frequency is reduced by opposing variations of the shunt section. A particular embodiment of the invention constructed and tested used a shunt section having a char acteristic impedance of 50 ohms and 270 electrical degrees in length at 885 megacycles. Short-circuited line sections of lengths equal to /4, /4, /4, etc., wavelengths and open-circuited lines of lengths 1, 1 etc, wavelengths are practical anti-resonant circuits which may be used. The particular examples given are merely illustrative and not intended to be restrictive. The actual impedance values and the line lengths used in any particular construction may vary in dependence upon the antenna impedance and the line impedance. The antenna impedance in turn is dependent upon the cavity construction and operating frequency band.

The mechanical details of the two-element matching section are shown in more detail in Figure 6. Herein the matching sections 55 and 5'! and the sleeve 20 associated with radiator 2| are shown in section. The matching section 55 may for example consist of a piece of coaxial line having its outer sheath 58 soldered or otherwise firmly electrically connected to the upper end of sheath 20. The inner conductor 59 of thematching section 55 is connected to the bottom end of radiator member 2 l. The shunt matching section 57 may consist of a piece -of coaxial line of a length so as to be anti-resonant-at the desired frequency with the inner conductor 6i and the outer sheath 62 connectedtogether'at one end by a short-circuiting means $3. At theother end outer shell 62 is electrically connected to-the upper end of sheath 23 while the inner conductor BI is connected to radiator member 2! in-paral-lel with conductor 59. The junction between-conductors 59, El and '21 maybe entirely surrounded by solid dielectric material -64 to seal the junction between the line sections and the base of radiator 2|. The dielectric material alsoserves as a support for the radiators. The dielectric material 64 is preferably polyethylene, a syn thetic insulating material used as a soliddielectric between the outer shell and the inner cenductor of most commercially obtainableicoa-xial transmission lines.

In Figure 7 the transverse section of the antenna-shows a modified form of radiator and sleeve-whereby the dimensions of the cavitymay bejf'urther reduced. "Radiator '21 and sleeve20" are semi-cylindrical in-section. *Thus,the axis of the radiator and sleeve may be-nearerthe mouth of cavity l2.

Typical radiation patterns of the antenna .of Figure 1 *areshown in Figures ;'7 andt8, F-igure showing by meansof radiation pattern J-O-the field strength in .a plane normaltotheconducitive sheet 10 and. normalsto theanten-na 2 I The plane of polarization :of the radiated energyis pa t e axis orthe an na WhileF nr v h w y m an of rad a n p ttern-Bl t e di tribution of energy in a plane normal to ground plane l0 and in a plane parallel ,to thefanten na 2|.

The distribution of radiation shown in Fig- .ures.7. and,8 may also be considered from the aspect of the installation of the antenna in ant airp ane- ;fiuppese the cav ty ube inounteqjon' the underside of the wing or fuselage of an airplane with the length of the cavity parallel to the line of flight, then the curve of Figure 7 is an athwartship pattern for horizontal polarization and the curve 80 of Figure 8 is the vertical foreand-aft pattern. It is evident from these figures that the radiation from a cavity antenna is entirely concentrated in the hemisphere toward,

which the aperture faces. The curves are predicated upon the assumption that the cavity is mounted in a surface having a radius of curvature not less than the length of an operating wavelength.

The impedance characteristics and field pattern characteristics of the antenna of the present invention make the antenna suitable for use in many applications. For example, in an altimeter installation two cavities may be used, mounted several wavelengths apart in the underside of a wing or fuselage with the cavity axes parallel, one cavity being used for transmission and the other for reception.

A single cavity mounted as mentioned above would be suitable for ultra high frequency guided missile control. Asingle cavity mounted in the tail of the plane with the aperture opening aft provides a very satisfactory pattern and ample band width for automatic tail-warning devices. Alternately, a single cavity mounted in the nose of the plane looking forward and slightly downward would be useful for identification systems. If it is desired to obtain a fairly complete coverage of space from an airplane a pair of antenna mounted on either side of the fuselage and fed in phase quadrature may be used while the addition of a third cavity mounted in the nose or the tail of the airplane, the three fed in a three-phase relationship, would result in a still more uniform pattern. Due to the compensation inherent in polyphase feed, these systems would have a greater band width than a single cavity.

As indicated above, the antenna of the present invention is usable for either transmitting or receiving service depending upon the type of radio frequency transducer means connected to the remote end of transmission line TL.

While I have illustrated a particular embodiment of the present invention, it should be clearly understood that it is not limited thereto since many modifications may be made in the several elements employed and in their arrangement and it is therefore contemplated by the appended claims to cover any such modifications as fall within the spirit and scope of the invention.

What is claimed is:

1. An ultra high frequency broad band antenna system including a conductive sheet having an aperture therein, a semicylindrical conductive structure arranged behind said aperture and connected to said sheet at its edges to define a hollow structure open at said aperture, a conductive sleeve member located within and having one end connected to said structure, said sleeve member extending concentrically of said structure, a conductive rod member arranged in end-to-end coaxial relationship to said sleeve member, and

means to extend a coaxial transmission line hav ing inner and sheath conductors within said sleeve member and to connect the respective conductors to said rod and said sleeve members at the adjacent ends thereof, the length of said structure being substantially three-eighths of the longest wavelength of the desired pass band, and the radius being between three-sixteenths and one-eighth of said longest wavelength, the total length of said rod and sleeve members being substantially three-sixteenths of said longest wavelength and the sleeve member having a length of substantially three thirty-seconds of said longest wavelength.

2. An ultra high frequency broad band antenna system including a conductive sheet having an aperture therein, a semicylindrical conductive structure arranged behind said aperture and connected to said sheet at its edges to define a hollow structure open at said aperture, a conductive sleeve member located within and having one end connected to said structure, said sleeve member extending concentrically of said structure, a con ductive rod member arranged in end-to-end coaxial relationship to said sleeve member, and means to extend a coaxial transmission line having inner and sheath conductors within said sleeve member and to connect said conductors respectively to said rod and said sleeve members at the adjacent ends thereof, the length of said structure being substantially three-eighths of the longest wavelength of the desired pass band, and the radius being between three-sixteenths and one-eighth of said longest wavelength, the total length of said rod and sleeve members being substantially three-sixteenths of said longest wavelength and the sleeve member having a length of substantially three thirty-seconds of said longest wavelength, and a further section of coaxial transmission line extending within said sleeve member and connected to said rod and sleeve members at the adjacent ends thereof, said coaxial transmission line section having a length at which adverse effects on bandwidth of said antenna are neutralized.

ROBERT S. WEHN'ER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,169,377 Walter Aug. 15, 1939 2,184,771 Roosenstein Dec. 26, 1939 2,212,214 Smith Aug. 20, 1940 2,238,438 Alford Apr. 15, 1941 2,239,724 Lindenblad Apr. 29, 1941 2,253,501 Barrow Aug. 26, 1941 2,258,953 Higgins Oct. 15, 1941 2,284,434 Lindenblad May 26, 1942 2,311,472 Roosenstein Feb. 16, 1943 2,400,867 Lindenblad May 21, 1946 2,414,266 Lindenblad Jan. 14, 1947 2,418,084 Montgomery Mar. 25, 1947 

